I’m at the Australian Institute of Sport in Canberra for the next few days, for the International Sport Nutrition Conference. Looks like some great sessions on tap from leading researchers around the world:

Plus lots of other talks and workshops. The first session starts in half an hour — I’ll try to post some updates over the next few days with highlights. It’s the first conference I’ve been to that started with a 7 a.m. run (during which, needless to say, I and few others got hopelessly lost, and almost got run over by a pair of kangaroos that bombed across the path about two feet in front of us).

I (and everyone I know) have always taken this for granted: if you weigh yourself before and after a workout, the difference tells you how much fluid you lost to sweat (after correcting for any water that you drank during the workout). If you lose more than about 2% of your bodyweight, dehydration will impair your performance. That’s what the ACSM guidelines on hydration say:

But it turns out there’s actually a hot debate currently raging in the literature about this. The latest salvo just appeared online in the British Journal of Sports Medicine, from researchers in South Africa (Pretoria, not Cape Town, though Tim Noakes is indeed listed as a co-author). They studied 18 soldiers doing a 14.6-kilometre march while drinking “ad libitum” (however much they wanted), and took careful measurements of a whole series of physiological parameters. One of those parameters is “total body water” — the sum total of all water stored in the body, typically totalling about 60% of body mass — which they measured using radioactive tracers. When we talk about hydration and dehydration, that’s what we’re really talking about: is there sufficient TBW to ensure that all the tissues and cellular processes in the body are working optimally.

The surprise: the subjects lost 1.98% of their body mass on average, but their total body water stayed roughly the same (actually, it increased by 0.53% on average). They drank 0.85 litres per hour, but sweated out 1.289 litres per hour. In other words, they were losing fluid — so how did their total body water stay the same or increase?

Some of the possible explanations are explored in this 2007 paper by Ron Maughan. One is “metabolic water”: when your body converts fat or carbohydrate into ATP, it release some water as part of the sequence of chemical reactions (one estimate is that it releases 0.13 g per calorie burned). A more significant possibility, especially for endurance athletes, is that every gram of glycogen you store ties up an estimated 3-4 g of water. A marathoner who carbo-loads and packs in 450 grams of glycogen, for example, could in theory have 1.35 kg of “hidden” water that will gradually be released into the body as carbohydrate stores are released during exercise.

So what this study claims is that these soldiers were sent out on a march and told to drink however much they wanted; they lost 2% of their body mass, but weren’t dehydrated. Their interpretation: the body’s thirst mechanism is built to maintain the osmolality (the concentration of “stuff,” essentially) in the blood and tissues, which was indeed preserved in this experiment.

Is the debate over? Far, far from it. For one thing, a laboratory experiment at Penn State published last year found exactly the opposite — that the amount of weight you lose during exercises correlates perfectly with the loss in total body water. How to reconcile these diverging views? I’m not sure, but I’m digging into the literature and doing some interviews for an upcoming article.

Actually, the title isn’t quite true — this is a just new wrinkle on the same Dutch study reported earlier this year that found no difference between platelet-rich plasma injections and placebo (saline) injections for 54 patients with chronic Achilles tendinopathy. The new paper, just published online at the British Journal of Sports Medicine, presents further data from this experiment: in addition to the previously reported pain scores (which, admittedly, are a bit wishy-washy and subjective), they used “ultrasonographic tissue characterization, a novel technique which quantifies tendon structure.”

Basically, they used ultrasonic imaging combined with computer image recognition to get an automatic (i.e. objective) measure of tendon health. The results: scores improved for both PRP and placebo (note that the subjects were also doing a rehab regime involving eccentric exercises during the study), but there was no significant difference between the groups.

Given the results presented earlier, this isn’t a big surprise — and of course, certainly isn’t proof that PRP doesn’t work in any context. But it’s another reason for skepticism. As the authors conclude, “these data argue against clinical use of this form of PRP in present clinical practice.”

I ran a trail race last weekend that involved a waist-high creek crossing through pretty cold water. Climbing the long uphill after the creek, my legs were suddenly dead — I felt like I could barely get my feet a few inches off the ground. So I sympathize with the volunteers in this study, published online ahead of print in the Journal of Strength and Conditioning Research by researchers at the University of Connecticut.

The researchers used vertical jump to measure leg power in a group of NCAA D1 athletes, with three main purposes: (1) to see how much power would increase after a dynamic warm-up, (2) to see how much power would decrease if the subjects were pre-cooled by standing waist-high in 12 C water, and (3) to see if the dynamic warm-up could off-set the negative effects of cooling — something that would be of interest to athletes who compete in cold temperatures.

Everything was pretty much as expected. The dynamic warm-up increased jump power by 5%, and the cold water decreased jump power by 21%. When the subjects did a dynamic warm-up after cold-water immersion, they regained 70% of the lost power — not perfect, but still good to know.

Leaving aside all this cold-water stuff, the main reason I’m posting this is highlight the ever-stronger consensus that dynamic warm-up is the way to go. As the researchers note in their introduction:

Traditionally, static stretching exercises have been used by many coaches to prepare athletes for sporting activity. However, studies have shown that static and proprioceptive neuromuscular facilitation (PNF) stretching may negatively impact jump performance and power output. Dynamic warm-up exercises now appear to be preferred after many studies have compared the 2 modes and demonstrated dynamic exercises to be much more effective.

So what does that mean in practical terms? Well, here’s the dynamic routine the researchers used:

Continuous warm-up 1 (20 yds)
1. Arm circles forward X 1: walking forward on the toes while circling the arms forward with the arms parallel to the ground
2. Backward heel walk w/arm circles backward X 1: walking backward on the heels while circling the arms backward with arms parallel to the ground
3. High knee walk: walking forward and pulling the knee up to the chest with both arms, alternates as you walk
4. High knee skip: skipping forward and bringing the knee up so that the quadricep is parallel to the ground
5. High knee run: running while focusing on bringing the knees up so that the quadricep is parallel to the ground
6. Butt kicks: running while bringing the heel to the glutes
7. Tin soldiers: walking forward and kicking a single leg up in front while keeping the knee locked in extension (alternates)
8. One leg SLDL walk forward X 1: walking forward with straight legs, lean forward on 1 leg and reach for the foot with the opposite hand
9. 1 Leg SLDL Walk Backward X 1: walking backward with straight legs, lean forward on 1 leg and reach for the foot with the opposite hand
10. Backward skip: moving backward and skipping at the same time
11. Backward run: running backward and extending the rear foot behind you
12. Back peddle: moving backward while shuffling the feet and keeping them low to the ground
13. Overhead lunge walk: hands on the head while doing walking lunges forward
14. Inchworm: starting in the push-up position, walk the feet into the hands; then walk the hands out to the push-up position

Following up on last week’s post about the increasing use of 3-D motion analysis, there’s a new Australian study examining the running stride of triathletes coming off the bike in this month’s Journal of Sports Sciences. They took “moderately trained” (i.e. club level) triathletes and measured their stride during a 30-minute, either with or without a bike ride beforehand, using EMG to measure muscle activation and motion capture to measure the stride.

The basic finding: 14 out 15 triathletes showed no difference in muscle recruitment between the two runs, but five of them did show kinematic differences (their joints were at different angles and moving differently). What’s surprising is that this is basically the opposite of what they found in a similar study of elite triathletes, who kept their stride pretty much constant but had different muscle recruitment patterns off the bike.

What this suggests is that it’s difficult to run “normally” coming off the bike, but elite triathletes have trained long enough to learn how to send different signals from their brain to their muscles in order to reproduce their normal stride.